4
That will largly depend on the modules you are using.
From this point forward, I would look very carefuly at the releases you download to be sure of compatability issues. Make sure you see 2.2+ in the release notes before considering a module/
Not all modules will work , however, they are all worth a try. I myself had no issues updating, but I'm not a huge fan of too many modules and general site clutter. If you are a minimalist like me, you will have very little problems, however that is no guarantee that all your modules will work out of the box, so to speak.
One thing you can do is to update each of your module templates, but only if you are haveing problems. Go on the premise, if it isn't broken, don't fix it.
You should be ok, but do be carefull about what blocks you place on your site because some changes may produce a blank screen because of one misconfigured block.
Rebuild your site layout one block/module at a time and you will be just fine.
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